1. Field of the Invention
The present invention relates to an optical exchange and an optical exchange method which are suitable for optical cross-connects, optical add/drop multiplexers, wavelength routers and etc. in high-speed large-capacity WDM (Wavelength Division Multiplexed) systems.
2. Description of the Related Art
Nowadays as the population of the Internet has spread explosively, traffic in the Internet is on the drastic increase. Wavelength division multiplexing (WDM) has been popular as one of the best means to build a large-capacity optical communications network.
FIG. 14 of the accompanying drawings schematically shows a conventional optical cross-connect (OXC) system for a trunk optical network according to WDM. The OXC system 100 comprises a plurality of optical switches 101-1 through 101-4 (only four depicted in FIG. 14) each connected to adjacent ones via optical fibers 102. Assuming that wavelength-division-multiplexed (WDM) optical signals are input to a particular one of the optical switches 101-1 through 101-4 via optical fibers, the particular optical switch cross-connects the input optical signals in terms of wavelengths and performs wavelength multiplexing over prospective output optical signals to be transmitted to the same destination switch, whereupon the resultant wavelength-multiplexed optical signals are transmitted to the selected destination switch.
If a trouble or fault has occurred in an optical fiber 102 in the OXC system 100 of FIG. 14, the system 100 automatically and instantly diverts the output optical signals onto a protection optical fiber (or another optical fiber) 102.
FIG. 15 schematically shows a conventional optical add/drop multiplexing (OADM) system 200. The OADM system 200 comprises a plurality of optical switches 201-1 . . . 201-5 (only five depicted in FIG. 15) connected one to another in a ring topology. Of the five optical switches, three switches 201-1, 201-3, 201-4 are each equipped with a router 202, 203, 204. And of the two remaining optical switches, one optical switch 201-2 is connected to another optical switch 201-6 and the other optical switch 201-5 is connected to a SONET (Synchronous Optical NETwork) system 205.
The OADM ring system 200 is used in a regional network or metropolitan area network (MAN), which is branched from a trunk network or wide area network (WAN); optical signals can be transferred from the ring line of the OADM system 200 to the regional network and vice versa without being converted between electric and optic for every wavelength. When the traffic at a point in the regional network, the OADM ring system 200 dynamically changes allocation of wavelengths to thereby automatically expand the wavelength band and hence to increase the transmission capacity so that the network configuration automatically varies to meet the local traffic in the regional network.
FIG. 16 shows the details of the individual optical switch 110 in the OXC system 100 or OADM system 200, comprising a first optical amplifier 111, an optical demultiplexer 112, an optical switch device 113, a variable attenuator 114, an optical multiplexer 115, a second optical amplifier 116, and a gain-level equalizer 117.
In the optical switch 110, the first optical amplifier 111 amplifies the optical signals that have lowered in level as transmitted over a long distance through an optical fiber. The resulting optical signals are demultiplexed by the optical demultiplexer 112 in terms of wavelengths. Then the optical switch device 113 performs a switching operation, such as cross-connector add/drop, on the demultiplexed optical signals.
And the variable attenuator 114 attenuates/equalizes power levels of the individual switched optical signals (e.g., in the ring line of FIG. 15) in terms of the wavelengths. The multiplexer 115 performs wavelength multiplexing on the attenuated/equalized optical signals. Then the wavelength-multiplexed optical signals are amplified by the second amplifier 116, and the individual gain levels of the resulting optical signals are equalized by the gain-level equalizer 117.
In short, the WDM system, such as the OXC system 100 in FIG. 14 and OADM system 200 in FIG. 15, transmits the wavelength multiplexed optical signals between long-distance end devices through the optical fiber. During this long-distance transmission, the power levels tend to stagger between the individual optical signals of every wavelength, causing not only a narrowed transmission band but also an impaired SNR (signal-to-noise ratio) the WDM system partly due to the lowered optical power level.
Further, the first and second amplifiers 112, 116 are exemplified by erbium doped fiber amplifiers (EDFA), whose gain band is approximately tens nm as a single amplifier is used. But, as a common knowledge in the art, when two or more EDFAs are used as connected one to another, a wavelength gain difference would occur because their wavelength-gain characteristics are emphasized, narrowing the gain band to the extremity. It is also known that, in optical amplification according to EDFA, the optical signals of the wavelength however lower in gain would be buried in naturally emitting incoherent adjacent optical signals of the wavelength higher in gain.
Evenness of gain characteristics of the first and second optical amplifiers 112, 116 as connected in series is therefore essential to compensate possible loss of the SNR. Generally, however, a wavelength-gain characteristic of EDSA has two peaks in the wavelength band and is hence uneven.
Meanwhile, the variable attenuator 114 attenuates/equalizes the staggered power levels of every wavelength shown in (a) of FIG. 17 to obtain the even power level shown in (b) of FIG. 17. The gain-level equalizer 117 (FIG. 6) is omitted here in FIG. 17 for clarity of description.
And the gain-level equalizer 117 equalizes the staggered power levels of the wavelength-multiplexed optical signals shown in (a) of FIG. 18 to the even power level shown in (c) of FIG. 18 by imposing an inverted characteristic of the gain of the amplifier 116 shown in (b) of FIG. 18 over the gain of the amplifier 116. The variable attenuator 114 (FIG. 16) is omitted here in FIG. 18 for clarity of description.
However, the optical switch 110 of FIG. 16 encounters the following problems because of the variable attenuator 114 and the gain-level equalizer 117.
First, the gain-level equalizer 117 may be realized by an optical filter, such as a Fabry-Perot-etalon filter or a Fiber Bragg Grating (FBG), disposed downstream of the second amplifier 116 as EDFA; because the wavelength-gain characteristic of every practical EDFA is originally complex, it is very difficult to design a filter having such a filter characteristic as to meet an inverted one of the complex EDFA characteristic.
Further, a somehow precise gain-equalization filter can be realized by combining two or more optical filters that correspond one to each of Fourier series terms, which are obtained by approximation. For an improved accuracy, however, it is necessary to connect plural optical filters in series, which would increase the loss of transmission as well as the system size.
Furthermore, the variable attenuator 114 can be realized by placing a Mach-Zehnder interferometer and a semiconductor optical amplifier (SOA) gate, which are to be controlled, respectively in two parallel waveguides for optical signals of the corresponding wavelengths. It is however difficult to compensate either possible wavelength dependency or deflection dependency due to the user of waveguides.
Because both the variable attenuator 114 and the gain-level equalizer 117 exist for every optical switch device 113, as shown in FIG. 16, it would result in an increased size of the whole system.